In entrained-flow gasification, the starting material to be gasified (e.g. coal dust) is kept available in the feed system, pressurized to the required operating pressure of, for example, 4 MPa and fed via feed lines to the burner. Here, the rapid pressurization of the fuel dust to the operating pressure in the transfer vessel presents a particular problem. Pressurization is effected by addition of N2 or CO2 as pressurizing medium. This pressurization process can lead to consolidation of the fuel dust bed when the pressurizing medium is introduced exclusively from the top into the transfer vessel. In order to ensure trouble-free running-out of the fuel dust, this consolidation must be avoided at all costs.
To avoid consolidation of the fuel dust bed in the transfer vessel as a result of the pressurization process, a substream of the gas required for pressurization has to be introduced via the lower, conical region of the vessel. Discharge aids as are routinely used in the form of, for example, ventilation cushions or vibration pads in silo technology are unsuitable. These elements are designed for the atmospheric-pressure range and are not subjected to any relatively great pressure stress or temperature-change stresses. Use has hitherto been made of elements which have to fulfill two tasks. Due to the requirement of a very short pressurization time of the transfer vessel, relatively large amounts of gas have to be able to be introduced via these elements into the transfer vessel within a few minutes. This requires a high porosity of the filter material to keep the pressure drop as low as possible. Backflow of fuel dust into the gas path has to be avoided. On the other hand, these elements serve as discharge aids during emptying of the transfer vessel, which requires a very large gas introduction area over which the gas flows uniformly distributed into the transfer vessel. Elements having a synthetic resin-bonded pebble filter have hitherto been used. Owing to its porosity, this material presents a very low flow resistance. At the same time, the porosity is chosen so as to be sufficiently low to avoid intrusion of fuel dust particles into the filter or into the gas feed line. Owing to the materials properties of the pebble filter, the maximum differential pressure over this element and the maximum pressurizing gas temperature are limited to appropriate values. This limitation of the differential pressure or of the temperature normally presents no problems in plant operation. When these parameters are exceeded as a result of incorrect operation, e.g. increased pressurizing gas feed or exceeding of the maximum permissible pressurizing gas temperature, damage to the synthetic resin-bonded pebble filter occurs.
Exceeding of the design parameters (differential pressure, temperature) can lead to damage to the pebble filter. A reduction in the filter material thickness leads to a reduction in the maximum permissible differential pressure. However, the maximum permissible differential pressure is likewise an important design criterion for the element.